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> I'm trying to figure out what tools are available for longitudinal   
> analysis of fMRI data, ideally using a linear mixed models approach   
> that is used for other longitudinal data.
I am not aware of sowftare implementing this in the voxelwise setting.  
If you could specify a ROI and extract data from there, you could of  
course use R to specify and test your model, using different random  
structures. I suspect you'd be better off with modelling the residuals  
with an autocorrelation structure. First, it is appropriate for the  
longitudinal setting, and second there is more of it in neuroimaging  
software.

> Additionally, how should  missing data be dealt with? Is there a way  
> to impute missing data  using a missing at random framework, or  
> similar?
This strikes me as a hard combination, and I'd be surprised (and  
interested) to hear there is anything for neuroimaging in this corner.  
You have to generate random data with the correlation struture implied  
by the model, which adds to the difficulty of imputing under the  
spatial correlation structure. There is a fairly recent book (Daniels  
& Hogan) on longitudinal models + missingness, where they propose to  
use freely available sofwtare (that you can call from R) for this  
purpose.

Best wishes,
Roberto Viviani
Dept. of Psychiatry, University of Ulm